It is well known that olefin will generate polymer with a relatively high molecular weight at a relatively low temperature, while generate polymer with a relatively low molecular weight at a relatively high temperature. On the other hand, the activity of catalyst is a function of the reaction temperature. If in one single reactor the reaction temperature is controlled to change the molecular weight of polymer obtained, the speed of polymerization reaction will be generally out of control. Therefore, no matter in the laboratory experiment or in the commercial application, the temperature in any type of polymerization reactors has to be maintained relatively constant. In particular, for olefin polymerization process in the commercial application, whether the polymerization reaction temperature is constant or not directly determines continuity of the process or possibility of any accident that would be caused.
Therefore, for a long time it is believed that an even distribution of temperature in a fluidized bed polymerization reactor is of vital importance for controlling the polymerization process and stabilizing the quality of polymers obtained. FIG. 1 schematically shows a conventional fluidized bed polymerization reactor 50, which includes a column 51 and a circulating unit 60 arranged outside of the column 51. A liquid distributor 52 is arranged inside the column 51, and divides the column 51 into an upper zone (i.e., reaction zone) 53 and a lower zone 54. The circulating unit 60 includes a hydrogen source 64, a compressor 62 and a cooler 63, which are connected together in series by means of a suitable line 67. The circulating unit 60 is connected to the column top at one end, and to the column bottom at the other end. In reaction, monomer is fed to the lower zone 54 from a feeding port 55, and, after passing through the liquid distributor 52, enter into the reaction zone 53, where the monomer begins to react in the presence of suitable catalyst. The gas product generated in the column 51 flows into the circulating unit 60 from the column top, and is mixed with hydrogen from the hydrogen source 64. Afterward, the mixture is compressed by the compressor 62 and cooled by the cooler 63, forming a gas-liquid mixture. The gas-liquid mixture is then delivered to the lower zone 54, and enters into the reaction zone 53 through the liquid distributor 52. The temperature in the reaction zone 53 of the column 51 is maintained constant. For example, in U.S. Pat. No. 4,379,758, U.S. Pat. No. 4,383,095 and U.S. Pat. No. 4,876,320 etc. a similar fluidized bed polymerization reactor is disclosed, which comprises a closed circulating loop consisting of a fluidized bed reactor, a circulating gas compressor and a circulating gas cooler. The circulating gas containing reactor monomer (e.g., ethylene, α-olefin), hydrogen and nitrogen is circulated in the loop, and the polymerization temperature in the whole reactor is controlled at a certain constant value. However, with one single catalyst and a constant polymerization temperature in single reactor, only polymer with a molecular weight distributed in a relatively narrow range will be generated by olefin polymerization.
In order to improve the physical property or processing property of the polymer products, researchers have been trying to develop advanced process and technology. In the conventional olefin polymerization reactor and the process thereof, typically two or more reactors connected with each other in series are used, so that the polymer generated by olefin polymerization has a bi-model or broad molecular weight distribution. That is to say, under different reaction temperatures or gas components, the olefin polymerization can generate polymer with different molecular weights, thus widening the distribution of molecular weight. For example, WO 2009/076733A1 discloses an olefin polymerization reaction is carried out in two reactors arranged in a serial configuration, wherein polymer with a relatively low molecular weight is generated at a relatively high temperature in the first reactor, while polymer with a relatively high molecular weight is generated at a relatively low temperature in the second reactor. However, with regard to this type of in-series reactors, the reaction process is complex, the equipment is costly, the operation is difficult, and the continuous stability is poor.